1. Field of the Invention
The present invention relates to a tracking control method in which a main deflector deflects an electron beam to ensure that a region in a subfield of a mask blank placed on the stage that continuously moves is irradiated with the electron beam in accordance with the movement of the stage and to an electron beam writing system having means for performing the tracking control method, and more particularly to a tracking control method and an electron beam writing system capable of avoiding an abnormality of tracking.
2. Background Art
In recent years, an electron beam writing apparatus has used a vector scanning method to improve the throughput of the apparatus. In the vector scanning method, an electron beam is deflected only to a region (on a mask blank) in which a pattern is to be written. Specifically, in the vector scanning method, a stage having the mask blank thereon is continuously moved, and tracking control is performed to ensure that a main deflector deflects the electron beam to irradiate with the beam a predetermined region in a subfield of the mask blank in accordance with the movement of the stage.
In this writing method, however, the relationship between the speed of the movement of the stage and the deflection range of the electron beam may be not appropriate. That is, an area irradiated with the electron beam deflected by a control system for the main deflector may deviate from the predetermined region, and the writing operation may be aborted for an abnormality.
Proposed heretofore is a tracking control method in which deflection data in accordance with the stage movement is input to a highly responsive sub deflector with a narrower deflection range than that of a main deflector, and the tracking control of an electron beam is performed at high speed and with high accuracy (refer to, for example, Japanese Patent Laid-Open No. 196394/1995 (Hei 06-196394) (claims 1 and 3)).
In the above case, since the sub deflector deflects the electron beam to a region smaller than a region to which the electron beam is deflected by the main deflector, there is a high possibility that an area to be irradiated with the deflected electron beam deviates from a desired region. To avoid this, a conventional electron beam writing apparatus has means for detecting the deviation. When the means detects that there is a possibility that the deviation occurs, a writing operation is temporarily stopped. The main deflector corrects a region to be irradiated with the electron beam and resets the region. The writing operation is then performed.
In the conventional tracking control method, however, there is a high possibility that the writing operation is stopped. As a result, the throughput of the electron beam writing apparatus is significantly reduced, and an operation for restarting the writing operation is required.
As another method for avoiding the abortion due to an abnormality of the writing operation, a method for the writing operation in which the speed of the movement of the stage is reduced has been proposed. When the speed of the movement of the stage is low, however, the throughput of the electron beam apparatus is also reduced. This is not compatible with the desired purpose of the vector scanning method.
As still another method for avoiding the abortion, the following method has been proposed: a method in which a subfield is divided into a plurality of sections, and the number of electron beam shots per subfield positioning operation is reduced.
In this method, it is necessary that a trial writing operation be performed to obtain an optimal number of divided sections before an actual writing operation.
FIG. 4 is a flowchart showing a writing process using the conventional method in which a subfield is divided into sections. In the writing process, shot data (Xm, Ym) that specifies the position (in X-Y coordinates) of a region to be irradiated with a beam is first read in step S′1 in order to perform a trial writing operation without a mask blank. Next, control data for controlling a main deflector is calculated based on the shot data in step S′2. In order to convert the control data into an analog signal, a digital-to-analog converter (DAC) is set in step S′3. After the time for settling a voltage to be applied to the main deflector elapses in step S′4, the trial writing operation (trial shot) is performed in step S′5. It is determined whether or not the number of divided subfield sections will cause abortion of a writing operation in step S′6. When the number of divided subfield sections is not optimal, that number is changed, and the above operations in steps S′1 to S′5 are performed again. When the number of divided sections is optimal, the writing process proceeds to an actual writing operation in step S′7.
In the actual writing operation, position data (Xm, Ym) that specifies the position (in the X-Y coordinates) of a region to be irradiated with a beam is read in step S′8. In addition, stage data that specifies the position (in the X-Y coordinates) of a moving stage is read in step S′9. When it is determined that the mask blank reaches a region in which a pattern can be written in step S′10, control data for controlling the main deflector is calculated based on the position data and the stage data in step S′11. In order to convert the control data into an analog signal, the DAC is set in step S′12. After the time for settling a voltage to be applied to the main deflector elapses in step S′13, a beam is shot in step S′14.
Then, the above operations in steps S′8 to S′14 are repeated until the entire subfield is irradiated with the beam in step S′15.
The above method for performing the trial writing operation involves complex steps and inconvenience in that the trial writing operation needs to be performed a plurality of times to find an optimal number of divided subfield sections.
During the tracking control time in the above method, a shot operation is not performed during the time period from the calculation of the control data for the main deflector, through the DAC setting, to the settling of the voltage to be applied to the main deflector. Therefore, the abortion may conceivably be avoided by reducing the time required for the above operations and thereby increasing the shot operation time.
However, the problem to be solved is not to cause discrepancies between the time period for the shot operation within a subfield and the speed at which the stage moves. When the shot density is remarkably high, the abortion may still occur even if the time period for the shot operation is increased. Thus, the above method also does not solve the problem essentially.